We have the D@#ned blue herons fishing ours out... Especially in the summer when the springs are running slow and it takes longer for them to refill if all the cows come to drink in a group. Which ours do... they tend to make a certain "trip" around the pasture and go to drink in a group around 10-11 and stay in the trees for a few hours, then go out to graze again... going down to drink again 6-8 p.m. Then they seem to stay out and graze most of the night. I had to scoop out the fish one time because the water was so low and I was having to haul water to the troughs up by the barn to supplement the spring. Had a bear tear up the float in one trough at a pasture and noticed all the big goldfish were gone... few small young ones darting around. Think it may have been doing some fishing as well as being destructive.